Field of the Invention
This invention relates to a pneumatic tire and more particularly to a pneumatic tire for suppressing or preventing the growth of cracks developing in tread grooves.
In general, with the pneumatic tire having a tread pattern of a rib type or a lug type, it has been experienced that there have inevitably developed a plurality of minute or hair cracks at the stress-concentrating points in the grooves due to injuries caused by stepping on a nail or a sharp edged stone during the service of a tire or due to the chemical deterioration or fatigue of tread rubber due to the action of ozone. More particularly, since a tension is applied to the bottom portions of tire grooves having a tread pattern of a rib type in the widthwise direction of a tire, i.e., in the direction of a rotary axis of a tire, there are produced stress concentrations at the ends of cracks, resulting in promoting the growth of cracks produced in the grooves in the lengthwise direction thereof. In this manner, the cracks grow up gradually with the use of a tire, eventually resulting in vital damages for a tire, such as tread separation, rib tearing, burst and the like.
There have been proposed many attempts which prevent the growth of cracks developing in tread grooves. The measures taken as those attempts are (1) considerations given to the composition of the tread material consisting of rubber composition so as to afford the crack-growth-resisting property thereto, (2) cyclical change in the level of the bottom portions of the grooves by providing corrugated bottom portions for the grooves A, as shown in cross section in FIG. 1 or projecting portions B of ribs which are formed on the side walls of grooves A and have obtuse corners as shown in FIG. 2, rather than the use of the ribs having acute corners, thereby avoiding the stress concentrations, and (3) considerations in design of the profile of the entire tire so as to avoid the stress concentration in the bottom portions of the grooves, etc. However, it was found that those attempts have met only partial success in preventng cracks.
It is a principal object of the present invention to provide a tire which prevents vital damages or injuries due to the growth of cracks created in the tread grooves.